It was possible to obtain relatively pure goethites (< z p) from a range of soils by dissolving the clay minerals in boiling NaOH. T h e goethites were analysed chemically and studied by X-ray diffraction. All goethites contained aluminium, the amount ranging from 15 to 3 0 mol. per cent. A100H. The ( 1 1 1 ) spacing decreased steadily with increasing aluminium content. A study of different particle sizes from one soil showed that the aluminium content increased with decreasing particle size, and the ( I I I) spacing showed a corresponding decrease. Extraction of soil goethite by free-iron removal agents, such as dithionite, brought into solution that aluminium contained in the goethites. The ease with which the goethite was dissolved b y such treatments was inversely related to the amount of aluminium in the goethite, and led to the surprising situation in one soil where the coarser goethite was more easily extracted than the fine goethite. Measurements of the (I I I ) spacings of many other soil goethites showed that almost invariably the unit cell dimensions were lower than those of pure goethite, indicating aluminium to be a very common replacement for iron in soil goethites. T h e degree of substitution of aluminium for iron probably depends on the weathering conditions in the soil. The aluminium probably restricts the size to which the goethite crystals can grow; it may, by its influence on particle size and by changing the composition of the goethite, alter the colour of goethite; it could modify the phosphate-fixing power and other chemical properties of soil goethites. IntroductionEXTENDED observations in this laboratory have shown that the X-ray
Abstract--Aluminum-substituted hematites (Fez-xAlxO3) were synthesized from Fe-A1 coprecipitates at pH 5.5, 7.0, and in 10 -1, 10 -2, and 10 -2 M KOH at 700C. As little as 1 mole % AI suppressed goethite completely at pH 7 whereas in KOH higher AI concentrations were necessary. AI substitution as determined chemically and by XRD line shift was related to AI addition up to a maximum of 16-17 mole %. The relationship between the crystallographic ao parameter and AI substitution deviated from the Vegard rule. At low substitution crystallinity of the hematites was improved whereas higher substitution impeded crystal growth in the crystallographic z-direction as indicated by differential XRD line broadening. At still higher AI addition crystal growth was strongly retarded. The initial AI-Fe coprecipitate behaved differently from a mechanical mixture of the respective "'hydroxides" and was, therefore, considered an aluminous ferrihydrite.
No abstract
Double-cation hydroxy compounds, generally with the pyroaurite-type structure, were synthesized by a reaction involving the hydrolysis of a cation in solution by a fully hydrolysed and precipitated hydroxide of a second metallic cation. Synthesis of this type of compound is not limited to reactions between di-and trivalent cations as previously thought but can, in this process, involve divalent-divalent, divalent-tetravalent and trivalent-trivalent cations. Crystalline compounds with reasonably sharp XRD patterns formed generally within a few hours at 25 ~ at pH 4-8.5 depending on the cations involved. The technique presents a more rapid method of formation of these compounds than previously described syntheses, and has been called induced hydrolysis because the fully hydrolysed cation causes complete hydrolysis of the second cation at a pH below that at which this would normally occur. The minerals takovite, Ni6A12(OH)I6CO 3 9 H20, and hydrotalcite, Mg6A12(OH)16CO 3 9 H20, were amongst some of the compounds formed and the reaction is considered to be more realistic in terms of conditions encountered in natural environments than previous syntheses. The reactions described could also be expected to affect the mobility of heavy elements in the environment. These double hydroxy compounds convert on heating between 300-500~ to double-metal oxides with the spinel structure.Over 40 years ago Feitknecht (1942) and colleagues described the formation of double-layer hydroxy compounds containing di-and trivalent metal hydroxides. These were generally formed by the co-precipitation at high pH of the hydroxides from a solution containing the two cations of differing valence, and aging the precipitate in the mother solution. Feitknecht recognized the importance of similar ionic radii of the metal cations in
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